<!DOCTYPE html>
<html lang="en">

<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>

  <style>
    @media (min-width:320px) and (max-width:375px) {
      .box {
        font-size: 15px;
      }
    }
    @media (min-width:375px) and (max-width:414px) {
      .box {
        font-size: 18px;
      }
    }
    @media (min-width:414px) and (max-width:480px) {
      .box {
        font-size: 21px;
      }
    }
    @media (min-width:480px) {
      .box {
        font-size: 24px;
      }
    }
  </style>
</head>

<body>
  <div class="box">
    我是BOX
  </div>
</body>

</html>